A bimetallic system is frequently superior in its behavior to a monometallic catalyst. The following effects may be considered to rationalize the existing data: formation of bimetallics but segregation occurs; particle size effect, dispersion; matrix effect, e. g. one component grafted to the support; change in hydrogen coverage; metal-support interaction (MSI); suppression of ageing effects, such as coke formation. These effects are discussed here for a series of examples, e. g. Ru-Cu, Ru-Au, Ru-Pt, Ru-Fe, Pt-Au, Pt-Ir, Pt-Fe bimetallic systems, considering the influence of the factors mentioned for different hydrocarbon transformations as well as for hydrocarbon synthesis.